The electronics industry continues to seek enhanced performance from products such as printed wiring boards used with integrated circuits in order to meet consumer demand for higher performance, lower cost computers and electronics equipment. Driving forces within the electronics industry include a desire for increased speed and functionality which requires reduced size of both components and interconnectors. Among the means by which interconnection circuitry can be reduced in size include the design of circuit boards that have smaller, finer lines and spaces to increase the line density on boards. Increasing line density leads to fewer circuit layers and smaller, lighter, electronic devices.
Printed wiring board circuitry can reside on a rigid fiberglass reinforced plastic or on flexible films to which are adhered metal foils used to form conductive circuit connections. The boards can contain interconnecting circuitry on one layer, two layers, or multiple layers. Boards with three or more layers can be manufactured using multiple two layer boards laminated together forming a multi-layer construction, or can be built up from a two layer board by sequentially adding dielectric layers and circuits.
Multilayer boards are most typically manufactured by laminating circuitized double sided boards in a stack using sheets of thermosetting polymers impregnated in fiberglass, known as pre-preg. Typically the outer most circuitized layers are added by using a sheet of pre-preg and a sheet of metal foil. The circuit connections on the double sided core circuits are usually manufactured by using subtractive techniques, while the outer metal conductor layer is shaped in a semi-additive method. The multiple circuit layers are electrically connected by mechanical drilling of holes through the board and plating with a conductive metal.
Sequential built-up boards are manufactured by laminating foils coated with thermosetting resins to a circuitized double sided board. The coated resin layer serves as a controlled dielectric layer separating the built-up circuit layer from the double sided circuit board. Electrical connections between circuit layers are made by plasma or laser drilling, followed by plating the resulting connecting hole. Circuitizing the built-up circuit is accomplished using semi-additive methods. Build-up layers can be stacked forming a circuit with many layers.
Circuit board designers require substrate materials on which extremely fine lines and spaces can be formed with a high degree of precision. Thin metal foils are generally a preferred substrate for the formation of circuit lines on circuit boards. The use of thicker foils results in greater waste of metal and reduces the line density that can be achieved. The metal films are most commonly formed by electrodeposition. Electrodeposited copper films typically must be of a defined minimum thickness,  greater than 1 xcexcm, to avoid holes or discontinuities.
Metal foils that are currently in use in the industry are typically at least 5 xcexcms in thickness. The use of thinner metal foil in printed circuit board would allow the formation of more densely packed lines and would reduce production costs. There is a significant interest in developing methods for obtaining thin copper foils. Existing methods for obtaining and placing a very thin metal foil on a laminate are limited.
U.S. Pat. No. 4,357,395 discloses a copper-clad laminate that is made by first forming a layer of silica on an aluminum carrier and then sputtering a copper film onto the silica layer. The copper layer is then laminated to a substrate, and the carrier and silica layer are mechanically stripped away, leaving the copper layer exposed. The examples disclosed that copper films 5-10 xcexcms in thickness were obtained by this method.
The method disclosed in U.S. Pat. No. 4,357,395 is not suitable for the manufacture of copper-clad laminates having an very ultra thin (0.1-0.3 xcexcm) copper foil, because very ultra thin foils are susceptible to picking when the carrier and silica layers are mechanically removed.
U.S. Pat. No. 4,431,710 reveals an aluminum carried copper foil where the copper was deposited onto the aluminum foil using vapor deposition at temperatures between about 100xc2x0 C. and 250xc2x0 C. Copper film of 5 xcexcms in thickness was demonstrated. After lamination to a substrate, the aluminum carrier is removed with a peel force of 0.5 lbs./in. to 2.0 lbs./in. The disclosure discusses the variability in the peel strength experiences by this method. The high peel strengths and variability would be detrimental to very ultra thin foils of less than 1 xcexcm in thickness. In addition, the exemplified foil is shown to have a significant impact on peel strength, dependent on contaminants.
U.S. Pat. No. 5,262,247 describes a foil consisting of a copper carrier layer, a chromate parting layer, a copper-nickel alloy layer, and finally a thin copper foil layer. Upon lamination the copper carrier plus chromate layer is removed. The resulting metallized substrate requires etching of the copper-nickel layer to reveal the copper layer. It is emphasized and claimed that the thin copper foil layer must be 1 xcexcm to 10 xcexcms in thickness. Below 1 xcexcm copper thickness is not used due to the etching step required to remove the copper-nickel layer. Etching the copper-nickel layer will also etch the underlying copper layer.
What is needed in the art is a method for obtaining very ultra thin metal foil laminates for the manufacture of printed circuit boards.
It is the object of the present invention to provide a metal-clad laminate for use in the manufacture of printed circuit boards, the laminate having a very ultra thin metal foil that will support the formation of very fine lines and spaces thereon.
Another object of the present invention is to provide a method for large scale production of a metal-clad laminate for the manufacture of printed circuit boards, the laminate having a very ultra thin metal foil that will support the formation of very fine lines and spaces.
It is a further objective of the invention to supply a foil for use in multi-layer and built-up circuit boards, the foil being very ultra thin to enable fine line circuit formation.
The present invention is a metal-clad laminate for the manufacture of printed circuit boards, the laminate having a very ultra thin metal foil that is at most about one xcexcm in thickness.
The present invention is also a method of manufacturing a metal-clad laminate for the manufacture of printed circuit boards, the laminate having a very ultra thin metal foil that is at most about one xcexcm in thickness.